Mixing dry and wet magmas in the lower crust of a continental arc: new petrological insights from the Bear Valley Intrusive Suite, southern Sierra Nevada, California

Exposures of arc crustal sections represent rare opportunities to directly evaluate lower magmatic processes and their link products in the middle upper crust. Within southernmost Sierra Nevada batholith, Bear Valley Intrusive Suite (BVIS) exposes a contemporaneously constructed ~ 30 km thick intrusive suite, thus is ideal for this type examination. Here we present detailed petrography mineral major trace element data BVIS. The deepest exposed portion BVIS (8–9 kbars) composed heterogeneous mafic igneous intrusions olivine metagabbro, olivine-hornblende orthopyroxenite, olivine-bearing hornblende norite, gabbronorite, hornblendite gabbro. Shallower (3–7 are comparatively homogeneous dominated by hypersthene-bearing hypersthene-free tonalites. Using amphibole-plagioclase geothermometry, show that crystallized over wide temperature range from 850 1070 °C, highlighting fractionation during isobaric cooling crust Sierran arc, while tonalitic liquids were emplaced at temperatures < 800 °C Calculated melt compositions equilibrium with amphibole gabbros similar measured tonalite bulk support generation tonalites through observed gabbros. Further, chemistry suggest multiple distinct crystallization sequences recorded different types gabbro, requiring presence coexisting parental melts contrasting H2O contents. available experimental data, develop model which mixing variably fractionated dry wet magmas viscosities followed crystallization-differentiation deep explain formation uniform shallower levels This process also explains unusual predominance orthopyroxene BVIS, limited aluminum enrichment compared differentiation hydrous basalts. Considering geochemical characteristics intermediate felsic rocks batholith Cascades, variable contents represents viable petrological produce SiO2-rich may be more common than previously recognized.